1. Field of the Invention
The present invention relates to an expandable ball seat for use in subterranean wells, in particular for wells used for production of hydrocarbons from subterranean formations.
2. Prior Art
To produce hydrocarbons, i.e. oil and/or gas, from subterranean reservoirs, a well extending through several strata of rock in a formation is provided. The well is usually constructed by drilling a borehole a distance through the rock, insert a steel casing into the wellbore and cement it to the formation. Cementing usually involves pumping wet cement down through a tubular liner and casing, out through a floating or sliding sleeve valve and up through the annulus formed by the formation and casing before the cement is left to set. The next section is drilled through the formation in the extension of the existing casing. A liner is then hung off by a liner hanger, and cemented to the formation. This process is repeated until a well comprising a number of sections with ever decreasing diameters have reached the desired depth. Typically one or more strata of the formation contain hydrocarbons, and typically the productive strata are separated by rock that does not carry oil or gas. To access the formation fluids, the casing may be penetrated at depths corresponding to the productive strata, and the formation may be hydraulically fractured to facilitate the flow of fluid from the formation into the production well. Horizontal wells may also branch out from a vertical production well, and extend several kilometers through a layer containing hydrocarbons.
A production pipe is typically provided within the casing or liner, and the completed well can be divided into several production zones by using packers. Valves control the flow of fluid during cementing. Other valves control fluid flow into a segment of production pipe corresponding to the production zone. In operation, fluid flowing from several zones at different rates can be mixed and conveyed up the production pipe to the surface.
To increase the amount and/or rate at which hydrocarbons are produced from a zone, various chemicals may be injected into the formation. The chemical may be water, brine, acids, solvents, surfactants etc, and it can be injected through the production well, or through one or more injection wells that may be provided at a distance from the production well. Thus, a typical oil or gas field can comprise one or more production wells, and zero or more injection wells. In some cases, an injection well may for example inject water or gas into or more zones to increase the pressure in the reservoir. Various additives to decrease the viscosity of fluid in a particular zone may also be injected. Such methods are collectively known as “stimulating a zone”, and their purpose is to enhance production from the zone. Particular methods for stimulating a zone are not part of the present invention, and hence not described in further detail in this disclosure. However, it should be understood that providing a larger number of injection points in a zone would help in distributing fluids and/or chemical additives in the zone.
It would also help production if the production well could be divided in more zones in a convenient and economical manner. For example, if water is injected through an injection well and breaks through the formation at some point along a horizontal branch of a well, the water content in the produced fluid could quickly exceed a predetermined level, and cause a decision to shut down that particular branch. However, there may be significant pockets of oil and gas left in the formation outside the region of water breakthrough. Hence, valves and packers may be provided to be able to isolate a certain section or zone in the horizontal well at a later time should this situation occur. On the other hand, the cost of valves and the time required to open a large number of valves when the zone is to be put into production can limit the number of sections or zones per branch. In turn, this might result in relatively large “dead zones” containing hydrocarbons that cannot be retrieved.
As indicated above, various mechanical devices are used during construction, completion and production, e.g. liner hangers, packers and valves of different sizes and designs. One of several ways to operate a mechanical device in a well is by using a drop ball, which are dropped or pumped with a fluid down into the well until it lands on a ball seat. Then, hydraulic pressure acts on the equipment and causes a relative movement between two parts, which movement activates the device. Devices activated by drop balls or other similar objects are comparatively inexpensive, and they do not require costly interrupts in the production, such as those caused when a working string or running tool must be run into the well.
The terms “drop ball” and “ball seat” are used for convenience herein, as drop balls are the most common means for activating devices in the well hydraulically. However, it should be understood that other equivalent objects used for the same purpose are considered as well. For example, a dart plug may be inserted in the fluid flow just ahead of the cement when a liner is to be cemented to the formation during completion of the well. The dart plug has a cylindrical body to separate the cement from the fluid below, and it typically has a rounded conical nose similar to a drop ball. When the dart lands on a seat connected to the valve, the fluid circulating through the liner is shut off, and hydraulic pressure builds up behind the dart. When the pressure reaches a predetermined level, shear pins or the like, which originally prevented relative movement between inner and outer sleeves of the valve, breaks. Then the sliding sleeve of the cementing valve slides downstream and opens the cementing valve, allowing cement to enter the annulus between the liner and the formation. After use, the dart may conventionally be broken, e.g. by a drill bit when the next section of the well is constructed. In a similar manner, a drop ball might have been used for the same purpose. Thus, the dart has the same function as a drop ball in that it lands on a seat, thereby closing fluid circulation such that hydraulic pressure can operate on a device.
A series of drop ball might be used to operate valves and other devices at different times and locations. For example, a series of drop balls may be used to operate a series of valves in different zones in order to open or shut off production from different zones in a production phase of a well. Conventionally, this is done by decreasing the diameters of the ball seats with increasing depth and using balls of different sizes. Then, a drop ball having a certain diameter will pass all the seats with larger diameters and land on the first seat having a diameter less than the diameter of the ball. Once the ball shuts off circulation, hydraulic pressure builds up behind it, and can be used to activate the device, e.g. by breaking a shear pin and/or provide some relative movement between parts within the device.
However, there is a limit to the number of different ball sizes that can be used in a well. If, for example, the diameters of two balls are too close to each other, even a minor piece of debris, sand or grit might cause a ball to land on a seat with a diameter slightly larger than the intended seat, thereby unintentionally activating the wrong device. Hence, there must be a minimum diameter difference between the drop balls to be used in a certain application. This also limits the number of pieces of equipment that can be operated by conventional drop balls or seats in a given well. Typically, about 20 or less ball seats may be used in one well for the reason discussed above.
As modern reservoirs increase in size and/or in depth, using drop balls in the conventional manner becomes impractical. For example, a well may extend 2000 meters or more vertically and/or horizontally. Using a maximum of 20 drop balls of different sizes, the mean distance between drop ball activated devices becomes 100 meters or more. This may exceed the thickness of a production zone. Because at least one valve should control the flow of fluid from each production zone, less than 20 ball operated valves will be available for use in horizontal wells branching out into the production zone(s).
In some cases it would be advantageous or necessary to provide more than one valve controlling the flow of formation fluid into the production pipe, limiting the number of ball activated valves available for other zones and/or horizontal wells even further. Using equipment operated by other means quickly becomes costly, as electrical motors need to handle the temperatures and pressures in deep wells, hydraulic lines become longer etc. Hence, it would be advantageous to provide a system wherein one drop ball could open an arbitrary number of valves in a vertical or horizontal well.
If there was a way to open an arbitrary number of valves by one drop ball in e.g. one production zone and/or horizontal branch, a large number of inexpensive valves could be installed. This would help when stimulating a zone in that fluid could be injected through a large number of injection points, and to drain the formation fluid into a production pipe through an arbitrary number of valves.
Thus, a main objective of the present invention is to provide an apparatus capable of activating an arbitrary number of drop ball operated devices using one drop ball only.
In particular, it would be feasible to install a greater number of valves to increase the number of injection points in an injection well. It would also be feasible to increase the number of valves in a long horizontal well, because all of them could be opened fast with one drop ball only. If, for example, water breaks through at a later time, a relatively small zone could be shut off, limiting the “dead zone” or pocket of hydro carbons that cannot be retrieved from the formation. Such a shutdown of certain valves could be done using methods known in the art, e.g. by providing the valves with standard fittings for conventional tools, and run a tool into the well by coiled tubing, slickline, a well tractor or running tool etc.